Field of the Invention
The present invention concerns an apparatus and a method for surveying at sea, in particular in a polar region under an ice cap.
Prior Art
Conventional seismic surveying at sea involves towing an array of cables, called streamers, behind a ship. The streamers are typically aligned parallel to each other, for example with a distance 25, 50 or 100 meters between each streamer, and they comprise at least one acoustic source capable of generating acoustic waves intended to propagate into a formation below the seabed, where the waves are reflected by different rock strata at different depths. The acoustic source is usually one or more airguns, but explosive charges or other signal sources are also known in the art.
FIGS. 4a and 4b are schematic views of an acoustic signal source.120′ with one or more airguns 440 arranged in clusters 44, which in turn are arranged in an array 450. The airguns 440 are supplied with pressurized air through lines 420. Each airgun may typically have a volume of 0,3-9,8 litres (20-600 in3), and a typical array may total up to about 50 litres (3000 in3). Each airgun in the array is charged with pressurized air through lines 420. Typical pressures are in the range 138-207 bar (2000-3000 psi). The air is released abruptly to create acoustic source waves. The characteristics of the source will be input to a mathematical model at a later stage. Optional shields 430, e.g. stainless steel plates, may be provided to protect a buoyancy element 410 from the full impact of the strong pulse generated when airguns are fired. The shields 430 and/or buoyancy element 410 must be able to withstand repeated loads as the airguns are fired at predetermined intervals during a seismic survey. Several seismic sources with some or all of the features above are available. Well proven sources are valuable due to the design, testing and adaptations required to make them work properly.
The reflected acoustic waves, or echoes, are received by acoustic receivers and recorded for further analysis. FIG. 3a schematically shows a grid on a seabed, where measuring points are spaced apart a distance X in one direction, and Y in a perpendicular direction. The devices 300 can be simple acoustic receivers or self contained nodes, each node comprising a receiver, a recorder and/or a power source. A full node is more expensive than a receiver, and the distances X, Y in a current array typically vary from 6.5 m between receivers to 300 m between full nodes. FIG. 4b is a side view of a cable 301 on the seabed. The receivers 302 are connected via the cable 301 to a common recorder 303, and the distance between the receivers may be as low as 6.5 m using current techniques for seismic surveying. Several alternative embodiments are known, for example receivers on the streamers, different types of receivers or nodes, etc. Selecting the equipment and where to deploy it is left to the skilled person.
The arrays in FIG. 3a and FIG. 3b are disposed on the seabed by a Remotely Operated Vehicle (ROV). Normally, there are at least two ROVs aboard a surveying ship for this purpose. A typical ROV is powered and/or controlled through a cable known as a ‘tether’. The tether extends from a mother vessel via a Tether Management System (TMS) to the interior of the ROV. The ROV may, for example, have one or two thrusters or propellers that are able to rotate about an axis perpendicular to the thrust force, and thus provide a thrust force having an adjustable angle relative to the body of the ROV. An ROV is standard equipment, and any suitable ROV can be used with the present invention.
In an entirely different type of survey, bathymetry, a side scan sonar can be used to provide signals that are echoed from the seabed. The signals are typically received, recorded and analyzed to map the topography of the seabed. Further, the signal sources and/or detectors may have known characteristics adapted to the mathematical models used for analysis.
In other types of survey, measurements may be taken at known points in space to map electro magnetic resistance, salinity, location and velocity of an ocean current, or any other parameter of interest.
In general, developing and testing equipment for use at sea is relatively expensive, and hence it is desirable to select well proven tools for use in a survey. However, selecting the actual equipment and techniques for measuring the parameters of interest is left to the skilled person.
Many of the known methods for surveying involve towing equipment at the surface of the sea, for example by providing a buoyancy element such as the element 410 in FIG. 4a, with sufficient positive buoyancy to keep the entire assembly 120′ floating.
However, towing equipment at or near the surface of the sea can be a problem in a polar region, where the water can be covered by solid ice or small and large pieces of ice may be floating in the water. For simplicity, the term ‘ice cap’ is used in the following description and claims to denote any solid ice, large and small ice floes and more or less broken ice floating in the water.
For surveying ice covered water, it has been proposed to use an icebreaker as a mother vessel in the survey. This causes new problems. For example, the propeller(s) and/or contact between the ice and the hull may induce noise in the acoustic signals of a seismic or bathymetric survey.
In order to prevent the noise from icebreaking from disturbing the signals, NO169743B proposes to use a conventional icebreaker as a towing vessel for a streamer cable, and stop the icebreaker during active survey. The streamer cable is hauled in with a speed corresponding to the desired propulsion speed of the cable during detection. After detection, the vessel again resumes ordinary operational speed, and the streamer is paid out with a speed which maintains the desired advancing speed of the system.
Further, using an icebreaker to create a passage for a single streamer cable towed behind the vessel can necessitate many passages through the ice to obtain a desired resolution in the survey. This, in turn, requires energy for breaking ice, and hence leads to a more expensive survey. Breaking ice for a vessel towing an array of several, parallel streamer cables implies added cost in a similar manner.
Still further, the time available from the ice breaks to the water refreezes may be short, and thus limit the time available for a conventional survey. Other problems associated with towing a cable through water with partly broken ice, include, for example, the risk for a cable being unintentionally deviated from its intended course, or even broken, by a floating piece of ice, e.g. an ice floe.
Similar problems are encountered in other surveys in polar regions, for example surveys using side scan sonar for bathymetry or electro magnetic resistance surveys.
WO 9912055 A1 og WO 9824685 A1 disclose other apparatuses and methods wherein streamers are towed behind a mother vessel.
US 2010226204 A1 discloses a method for seismic surveying wherein several parallel streamers are towed behind a mother vessel. Autonomous or remotely controlled vehicles can be attached to the distal ends of the streamers, such that the streamers can be controlled individually in a lateral and vertical direction. The streamers can be towed under ice and debris floating on the surface. Features from US2010226204 A1 appear in the preambles of the independent claims.
Surveying under an ice cap, e.g. solid ice or ice floes, still poses the problems discussed above, for example requiring breaking ice in order to tow the streamers behind the mother vessel.
Thus, an objective for the present invention is to solve at least one of the problems above, while using as much as possible of well proven techniques and equipment.